The present invention relates generally to systems for controlling motor driven surgical cutting instruments, and more specifically to such systems for controlling the position of the surgical cutting instrument upon stoppage of the drive motor. More particularly, the invention relates to controlling a brush type DC motor used to drive the cutting instrument.
Minimally invasive surgical cutting instruments are known and widely used to excise and remove biological tissue. Such instruments typically include a handpiece comprising a cutting tool housed within an elongated cannula, wherein the tool is disposed adjacent to an opening at or near the tip of the cannula. The cannula itself is configured for percutaneous insertion into a body via a small incision, and is manually maneuvered into position for tissue excision and removal.
Various configurations of cutting tools are known and may be driven manually, pneumatically or via an electrically controlled drive motor. In any case, tissue adjacent to the opening near the tip of the cannula is typically excised by driving the cutting tool with either a rotary or reciprocal motion relative to the cannula, whereby tissue is drawn into the opening (typically via vacuum) and excised by the cutting tool.
While motor driven surgical cutting instruments of the type just described have been widely used in surgical applications, many presently available designs suffer from a variety of drawbacks. For example, if the position of the cutting tool is not controlled when the drive motor is disabled, there exists a possibility that the cutting tool may come to rest in a position that traps or pinches unexcised tissue between a cutting surface of the cutting tool and the opening near the tip of the cannula. To avoid this problem, surgeons must typically maintain activation of the drive motor as the tip of the instrument is moved or removed from the surgical site, thereby compromising the accuracy and precision of the procedure. The foregoing drawback becomes more problematic as the complexity of the procedure increases, and is of particular concern when performing delicate procedures such as removing vitreous tissue during ocular surgery.
Designers of such surgical cutting instruments have attempted to address the foregoing problem by providing various systems for controlling motor position when stopping or disabling the drive motor. An example of one such system for controlling the position of a three-phase brushless DC motor is given in U.S. Pat. No. 5,602,449 to Krause, et al. The Krause et al. disclosure discloses an elaborate control system including multiple sensors for determining motor armature position at 6xc2x0 intervals. As is known in the art, brushless DC motors are typically speed driven rather than torque driven and accordingly have little rotational resistance associated with the operation thereof. Controlled stoppage of such a motor is thus extremely difficult, if not impossible, when the motor is operating at a high rotational speed, and the Krause et al. system is accordingly responsive to a motor stop signal to first decrease motor speed below some threshold speed level and then perform a controlled stop based on armature position.
Brushed DC motors, as compared with brushless DC motors, are typically torque driven rather than speed driven, and accordingly have a substantial rotational resistance associated therewith. Thus, while the Krause et al. system may effectively provide for controlled stopping of a surgical cutting instrument driven by a brushless DC motor, such elaborate control techniques are unnecessary when driving a brushed DC motor. What is therefore needed is a simple and inexpensive control technique for controlling the stop position of a brushed DC motor driven surgical instrument. Ideally the control system should be operable to control the position of the cutting tool when the drive motor is turned off so that unexcised tissue is not trapped between the cutting tool and the opening near the tip of the cannula.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, a tissue cutting apparatus comprises an elongated housing configured for insertion into a body site, the housing having a first end defining an opening adjacent thereto. A tissue cutting tool is disposed within the housing and defines at least one cutting surface configured for movement relative to the housing adjacent the opening to thereby excise tissue extending into the opening.
In one feature of the invention, a brushed DC motor drives a drive shaft coupled to the cutting tool. While the brushed motor is rotary, the cutting tool can be driven in a rotary or reciprocating fashion with an appropriate transmission mechanism between the drive shaft and cutting tool. Means are provided for sensing a predefined position of the drive shaft relative to the housing and producing a stop position signal corresponding thereto. In one embodiment, the sensing means is a Hall effect sensor and magnet arranged between the motor and the cutting tool.
A motor control circuit is provided to control operation of the motor. In one embodiment, the circuit includes a pulse-width-modulated controller. In another embodiment, the motor control circuit utilizes a current feedback to control the motor current. With either embodiment, the motor control circuit is operable in response to a motor stop signal to stop the motor with the drive shaft at the predefined position. Certain features of the motor control circuit are configured to take advantage of the natural braking characteristics of the motor and cutting tool in order to stop the motor at the appropriate position. This predefined position is arranged to orient the cutting surface of the cutting tool relative to the opening to avoid trapping unexcised tissue between the cutting surface and a boundary of the opening.
In one embodiment of the invention, the motor control circuit includes an op amp driven current feedback loop to control the current provided to the brushed motor. A switch is interposed in the feedback loop to disable the current feedback signal when it is desired to stop the motor. Eliminating the current feedback improves the braking characteristics of the motor and handpiece so that the cutting blade can be accurately stopped in its predetermined position. With this embodiment, the control circuit can respond to a motor stop signal to activate the feedback loop switch. The motor is powered until the stop position signal is received indicating that the drive shaft is at the predetermined position, at which point current to the motor is ceased to stop the cutter.
In another embodiment, the control circuit is responsive to a motor stop signal to drive the motor at a predefined motor speed less than the cutter operating speed. The control circuit is also responsive to a second occurrence of the stop position signal to deactivate the motor with the drive shaft at the predefined position.
One object of the present invention is to provide an improved surgical cutting apparatus that is operable to avoid trapping unexcised tissue between the cutting tool and a boundary of the opening in the cannula when the cutting tool drive motor is deactivated.
Another object of the present invention is to provide such improvements to surgical cutting apparatuses that have either a rotating cutting tool or a linearly reciprocating cutting tool.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.